Kinetics of Racemization and Dissociation of Tris(1,10-phenanthroline)iron(II). Hydrophobic Interaction between the Complex Cation and Alkylammonium Cations

Abstract

Abstract The rates of racemization and dissociation of [Fe(phen)3]2+ were determined in the presence of series of alkali-metal bromides, symmetrical tetraalkylammonium bromides, and alkyltrimethylammonium bromides. The alkali-metal bromides did not affect the racemization rate, but they decreased the dissociation rate. The retarding effect increased with increasing bromide concentrations and decreased in the order of Li+>Na+>K+> Rb+>Cs+. The alkylammonium bromides increased the racemization rate up to three fold and decreased the dissociation rate down to half the rate in pure water; alkylammonium ions with longer alkylchains induced greater changes of the rates. The effect of alkylammonium ions on the reaction rates was analyzed in terms of the formation of 1:1 aggregates of the complex cation with the alkylammonium cation caused by hydrophobic interactions. The equilibrium constant for the aggregate formation was greater for the alkylammonium ions with longer alkyl chains. The reaction rate constants of the hydrophobic aggregates remain almost constant with the increase in the alkyl chain length. The rate behaviors of the hydrophobic aggregates are discussed in terms of the reaction mechanisms of racemization and dissociation of [Fe(phen)3]2+.